Pneumatic tire

ABSTRACT

A pneumatic tire comprises the following belt (A) or belt (B). The belt (A) comprises a ply made of waved monofilament steel cords and a ply made of multi-filament steel cords, wherein the waved monofilament steel cord is (1) spirally waved so that the wave pitches PA are in a range of from 14 to 50 mm and the amplitude HA of each wave is in a range of from 0.002 to 0.02 times the pitch PA of the wave, or (2) two-dimensionally waved so that the wave pitches PB are in a range of from 5 to 50 mm, and the amplitude HB of each wave is in a range of from 0.002 to 0.05 times the pitch PB of the wave. The belt (B) comprises two plies made of waved monofilament steel cords, wherein the waved monofilament steel cord is waved so that the wave pitches P are in a range of from 0.008 to 0.08 times the circumference of the tire and the wave height of each wave is in a range of from 0.002 to 0.025 times the pitch P of the wave.

[0001] The present invention relates to a pneumatic tire more particularly to a radial tire having an improved belt structure being capable of reducing the tire weight.

[0002] Multi-filament cords made of a plurality of steel filaments twisted together are widely used in the belts of belted radial tires.

[0003] In recent years, in order to reduce the tire weight and cost, a belt composed of monofilament steel cords has been proposed.

[0004] However, in comparison with multi-filament cords, the monofilament cords are high in the bending rigidity and inferior in the fatigue resistance and durability. As a result tire performance such as ride comfort, steering stability during cornering and the like is not good.

[0005] It is therefore, an object of the present invention to provide a radial tire improved in the durability, ride comfort, steering stability and the like, while achieving weight reduction and cost reduction.

[0006] According to one aspect of the present invention a pneumatic tire comprises a carcass extending between bead portions through a tread portion and sidewall portions, and a belt disposed radially outside the carcass in the tread portion, the belt comprising a monofilament cord ply made of waved monofilament cords and a multi-filament cord ply made of multi-filament cords. Preferably, each waved monofilament steel cord has a cross sectional area of from 0.09 to 0.30 sq.mm. The waved monofilament steel cord is (1) spirally waved so that the wave pitches PA are in a range of from 14 to 50 mm and the amplitude HA of each wave is in a range of from 0.002 to 0.02 times the pitch PA of the wave, or (2) two-dimensionally waved so that the wave pitches PB are in a range of from 5 to 50 mm, and the amplitude HB of each wave is in a range of from 0.002 to 0.05 times the pitch PB of the wave.

[0007] According to another aspect of the present invention a pneumatic tire comprises a carcass extending between bead portions through a tread portion and sidewall portions, and a belt disposed radially outside the carcass in the tread portion, the belt comprising two monofilament cord plies made of waved monofilament cords. Preferably, the waved monofilament steel cord is waved so that the wave pitches P are in a range of from 0.008 to 0.08 times the circumference of the tire and the wave height of each wave is in a range of from 0.002 to 0.025 times the pitch P of the wave. The waved monofilament steel cord has an average thickness of from 0.13×10⁻³ to 0.28×10⁻³ times the circumference of the tire.

[0008] Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings.

[0009]FIGS. 1A, 1B, 1C and 1D are cross sectional views of pneumatic tires according to the present invention.

[0010]FIG. 2 is a cross sectional view of an example of the belt.

[0011]FIGS. 3A, 3B and 3C are cross sectional views each showing an example of the multi-filament cord.

[0012] FIGS. 4-13 each show an example of the waved monofilament cord and the amplitude and pitch of the wave thereof.

[0013]FIG. 14 is a cross sectional view of another example of the belt.

[0014]FIG. 15 shows a waved monofilament cord used in the belt shown in FIG. 14 and the wave height and pitch thereof.

[0015]FIG. 16 is a schematic cross sectional view for explaining the average thickness of a waved monofilament cord.

[0016]FIG. 17 is a cross sectional view of still another example of the belt.

[0017]FIGS. 18 and 19 each show a waved monofilament cord usable in the belt shown in FIG. 17 and the wave height and pitch thereof.

[0018] In FIGS. 1A, 1B and 1C showing a passenger car tire, light truck tire and truck-bus tire, respectively, and also in FIG. 1D, each tire according to the present invention comprises a tread portion 2, a pair of sidewall portions 3, a pair of bead portions 4, a carcass 6 extended between the bead portions 4, and a belt 7 disposed radially outside the carcass 6 in the tread portion 2.

[0019] The carcass 6 comprises at least one ply of cords arranged at an angle of from 75 to 90 degrees with respect to the circumferential direction of the tire and extending between the bead portions 4 through the tread portion 2 and sidewall portions 3 and turned up around a bead core 5 in each bead portion 4 from the axially inside to the outside of the tire to form a pair of turned up portions 6B and a main portion 6A therebetween. For the carcass cords, organic fiber cords, e.g. nylon, rayon, polyester, aromatic polyamide and the like, composed of a number of filaments twisted together are suitably used. But, multi-filament cords composed of a plurality of steel filaments twisted together may be also used.

[0020] Between the main portion 6A and turned up portion 6B in each bead portion, there is disposed a bead apex 8 made of hard rubber extending radially outwardly from the bead core 5, while tapering towards its radially outer end.

[0021] In the passenger car tire shown in FIG. 1A, the carcass 6 is composed of a single ply and the belt 7 is composed of two plies 21 and 20: a monofilament cord ply 20 and a multi-filament cord ply 21. In the light truck tire shown in FIG. 1B, the carcass 6 is composed of a single ply and the belt 7 is composed of three plies 21, 21 and 20: two multi-filament cord plies 21 and a monofilament cord ply 20. In the truck-bus tire shown in FIG. 1C, the carcass 6 is composed of a single ply and the belt 7 is composed of four plies 21, 21, 21 and 20: three multi-filament cord plies 21 and a monofilament cord ply 20.

[0022] In each example shown in FIGS. 1A, 1B and 1C, the belt 7 comprises at least a monofilament cord ply 20 and a multi-filament cord ply 21 as shown in FIG. 2. And the monofilament cord ply 20 is provided as the radially outermost ply.

[0023] The multi-filament cord ply 21 is composed of multi-filament steel cords 11 laid at an inclination angle of from 15 to 70 degrees with respect to the circumferential direction of the tire, and each multi-filament steel cord 11 is made of a plurality of steel filaments 11 a twisted together. In FIG. 2, a cord 11 made of five filaments 11 a which are twisted together, that is, a cord 11 having a 1×5 structure is used. But, various structures such as 1×4, 1×3, 1×2 structures shown in FIGS. 3A, 3B, 3C, respectively, and further layered structures (not shown) comprising a core filament and at least one sheath made of filaments surrounding the core filament, and the like can be used.

[0024] The monofilament cord ply 20 is composed of monofilament steel cords 10 laid at an inclination angle of from 15 to 30 degrees with respect to the circumferential direction of the tire.

[0025] By combining the monofilament cord ply 20 with the multi-filament cord ply 21, advantageous effects of the monofilament cords 10 can be obtained, that is, the cord rigidity and belt rigidity can be increased although the total weight of the cords is decreased, therefore both the tire weight and steering stability can be improved, and thus running performance can be improved while maintaining ride comfort. Such effects becomes maximum when the monofilament cord ply 20 is provided as the radially outermost ply.

[0026] For the monofilament cords 10, a steel filament whose cross sectional shape is a circle or an oval or a rectangle of which corners are rounded is suitably used. In any case, the cross sectional area S of the filament or cord is set in a range of from 0.09 to 0.30 sq.mm, preferably 0.09 to 0.25 sq.mm. If the cross sectional area S is less than 0.09 sq.mm, the cord rigidity decreases and the cornering power becomes insufficient and it is difficult to maintain steering stability. If the cross sectional area S is less than 0.30 sq.mm, the cord rigidity becomes too high, and it is difficult to provide ride comfort. Further, the cord tends to decrease its durability.

[0027] In this example, further, the total cross sectional area of the cords within a unit width of the monofilament cord ply 20 is set in a range of not more than 0.85 times the total cross sectional area of the cords within the unit width of the multi-filament cord ply 21. In case the cord count is constant in the widthwise direction of the ply, the product S×N of the sectional area S of a cord and the cord count N (/5 cm) can be used instead of the total cross sectional area. Here, the direction along which the unit width is measured and the direction in which the cords are count are at a right angle to the longitudinal direction of the cord.

[0028] The monofilament cord 10 is waved three-dimensionally (3-D) or two-dimensionally (2-D), whereby the fatigue resistance of the cord is improved, and it becomes possible to adjust the elongation characteristic.

[0029] In case of a monofilament cord 10A having a circular sectional shape, spiral (3-D) waving or 2-D waving can be made.

[0030] In case of spiral waving, it is possible to use not only a normal spiral without bend of which locus is, as shown in FIG. 4, a circle when viewed in the longitudinal direction, but also a polygonal spiral with bends of which locus is, as shown in FIG. 5, a polygon when viewed in the longitudinal direction.

[0031] In case of polygonal spiral waving, the filament may be bent such that longitudinal parts 15B and inclined parts 15A are alternately formed along the length, wherein each longitudinal part 15B extends substantially parallel to the longitudinal direction at one of the vertexes of the polygon, and each inclined part 15A extends between the adjacent longitudinal parts 15B.

[0032] In any case, it is preferable that the spiral pitches PA are set in a range of from 14 to 50 mm, and the amplitude HA of a wave is in a range of 0.002 to 0.02 times the pitch PA of the wave.

[0033] On the other hand, in case of 2D-waving, a curved wave without bend such as sine curve as shown in FIG. 6, and a bent wave with bends such as zigzag as shown in FIG. 7 can be used. Preferably, the pitches PB of the 2D-waves are set in a range of from 5 to 50 mm, and the amplitude HB of a wave is set in a range of from 0.002 to 0.05 times the pitch PB of the wave.

[0034]FIG. 8 shows a 3-D waving other than the spiral waving, wherein the filament 10A is waved in two orthogonal directions F1 and F2. In a plane parallel to the direction F1, the pitches PB are a constant value in a range of from 5 to 50 mm, and the amplitude HB are a constant value in a range of from 0.002 to 0.05 times the pitch PB. In a plane parallel to the direction F2, the pitches PB2 are a constant value in a range of from 5 to 50 mm but different from the pitch PB, and the amplitude HB2 are a constant value in a range of from 0.002 to 0.05 times the pitch PB2 but different from the amplitude HB.

[0035] Besides the above-mentioned monofilament cord 10A whose cross sectional shape is a circle, a monofilament cord 10B whose cross sectional shape is an oval or a rectangle of which corners are rounded as shown in FIGS. 9-13, can be suitably used, wherein the aspect ratio Dh/Dw of the cross sectional shape is preferably set in a range of from 0.65 to 0.95.

[0036] In a belt ply, such monofilament cords 10B are disposed so that their minor axes extend along the thickness direction of the belt ply 20 or the radial direction of the tire. Therefore, in comparison with the circular cross sectional shape, the inplane rigidity of the belt 7 is increased, but the out-of-plane rigidity is decreased. As a result, the cornering power can be increased to improve the steering stability, without deteriorating ride comfort. If the aspect ratio Dh/Dw is less than 0.65, the strength is liable to become insufficient.

[0037] In the same manner as explained above, such a low aspect ratio monofilament cord 10B can be waved spirally or two-dimensionally as follows:

[0038] In the spiral waving, a normal spiral without bend as shown in FIG. 9, and a polygonal spiral with bends as shown in FIG. 10 can be used. Preferably, the spiral pitches PA are set in a range of from 14 to 50 mm, and the amplitude HA of a wave is set in a range of from 0.002 to 0.02 times the pitch PA of the wave. In the 2D-waving, a curved wave without bend such as sine curve as shown in FIG. 11 and a bent wave with bends such as zigzag as shown in FIG. 12 can be used. Preferably, the pitches PB of the 2D-waves are set in a range of from 5 to 50 mm, and the amplitude HB of a wave is set in a range of from 0.002 to 0.05 times the pitch PB of the wave. Usually, the 2-D waving is made within a plane parallel to the minor axis as shown in FIGS. 11 and 12. But, it may be also possible to make the 2-D waving in a plane parallel to the major axis. Further, as explained in the monofilament cord 10A having a circular cross section, the orthogonal waving is also possible as shown in FIG. 13. The amplitudes HB and HB2 and the pitches PB and PB2 are limited as explained above.

[0039] On the other hand, in order to further improve the high-speed durability of the belt, a band (not shown) which covers at least edge portions of the belt 7 can be disposed radially outside the belt 7. Here, the band is a cord layer made of at least one cord spirally wound, or parallel cords laid at substantially parallel to the tire circumferential direction, that is, a cord layer of which cord inclination is about 0 to 5 degrees with respect to the circumferential direction of the tire. For example, a band composed of a single band ply of spirally wound nylon fiber cords is used.

[0040] If the pitches PA are less than 14 mm or the pitches PB and PB2 are less than 5 mm, the durability of the cord decrease. If the amplitudes HA, HB and HB2 are more than 0.02×PA, 0.05×PB and 0.05×PB2, respectively, the strength of fatigue resistance of the cord are liable to decrease.

[0041] Comparison Test 1

[0042] Passenger car tires (size 205/65R15) shown in FIG. 1A having specifications shown in Tables 1 and 2, and light-truck tires (size 205/85R16 117/115L) shown in FIG. 1B and truck-bus tires (size 11R22.5) shown in FIG. 1C having specifications shown in Table 3 were made and tested for durability, steering stability and ride comfort.

[0043] The results are shown in Tables 1, 2 and 3.

[0044] 1) Tire Weight

[0045] The tire weight is indicated by an index based on the prior art tire being 100.

[0046] 2) Durability Test

[0047] In case of passenger car tires, 8-turn (diameter 14 meters) was made 500 times, using a 3000 cc passenger car provided on all the wheels with the test tires. (Wheel rim size 15×6JJ, Tire pressure 200 kPa)

[0048] In case of light-truck tires, a light-truck provided on all the wheels with the test tires was run on bad roads for 20,000 km. (Wheel rim size 16×6K, Tire pressure 600 kPa)

[0049] In case of heavy duty tires, a truck provided on all the wheels with the test tires was run on bad roads for 20,000 km. (Wheel rim size 22.5×8.25, Tire pressure 800 kPa)

[0050] Then, the test tires were cut-open inspected for broken belt cords, and the number of broken points was counted. The results are indicated by an index based on the prior art tire being 100.

[0051] 3) Steering Stability Test

[0052] The test cars (passenger car, light-truck and truck) were run on a dry asphalt road of a tire test course, and the test driver evaluated the steering stability into ten ranks from the steering response, rigidity, road grip and the like. The larger the rank number, the better the steering stability.

[0053] 4) Ride Comfort Test

[0054] The test cars were run on dry rough roads of the tire test course (including uneven asphalt road, stone-paved road and graveled road), and the test driver evaluated ride comfort into ten ranks from the harshness, thrust, damping and the like. The larger the rank number, the better the ride. TABLE 1 Tire prior 1 Ref.1 Ref.2 Ex.1 Ex.2 Ex.3 Ex.4 Ex.5 Tire size 205/65R15 205/65R15 205/65R15 205/65R15 205/65R15 205/65R15 205/65R15 205/65R15 Belt 2 plies 2 plies 2 plies 2 plies 2 plies 2 plies 2 plies 2 plies Outer belt ply Monofilament cord Cord structure 1 × 5 × 0.25 1 × 1 × 0.65 1 × 1 × 0.42 1 × 1 × 0.38 1 × 5 × 0.25 1 × 1 × 0.42 1 × 1 × 0.42 1 × 1 × 0.42 Sectional shape — circle circle circle circle circle circle circle Sectional area S (sq.mm) — 0.332 0.139 0.113 0.139 0.139 0.139 0.139 Waving — spiral spiral spiral spiral spiral spiral 2-D Bend — none none none none none present none Pitch PA (mm) — 30 20 19 20 20 20 — Amplitude HA (mm) — 0.28 0.5 0.17 0.18 0.18 0.18 — HA/PA — 0.009 0.025 0.009 0.009 0.009 0.009 — Pitch PB (mm) — — — — — — — 20 Amplitude HB (mm) — — — — — — — 0.18 HB/PB — — — — — — — 0.009 Bending rigidity M (g cm) 207 42 29 42 42 42 42 Cord count N (/5 cm) 26 40 44 40 40 40 40 SXN (cord weight index) — 8.632 5.56 4.972 5.56 5.56 5.56 5.56 MXN (cord rigidity index) — 5382 1680 1276 1680 1680 1680 1680 Inner belt ply Multi-filament cord Cord structure 1 × 5 × 0.25 1 × 5 × 0.25 1 × 5 × 0.25 1 × 5 × 0.25 1 × 1 × 0.42 1 × 5 × 0.25 1 × 5 × 0.25 1 × 5 × 0.25 Sectional area S (sq.mm) 0.245 0.245 0.245 0.245 0.245 0.245 0.245 0.245 Bending rigidity M (9 cm) 28 28 28 28 28 26 28 28 Cord count N (/cm) 35 35 35 35 35 35 35 35 SXN (cord weight index) 8.575 8.575 8.575 8.575 8.575 8.575 8.575 8.575 MXN (cord rigidity index) 980 980 980 980 980 980 980 980 Tire weight 100 102 97 94 95 96 96 96 Durability 100 104 200 96 88 85 87 86 Steering stability 7 9 7 8 8 9 9 9 Ride comfort 7 4 8 8 8 8 8 8

[0055] TABLE 2 Tire Ref. 3 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Tire size 205/65R15 205/65R15 205/65R15 205/65R15 205/65R15 205/65R15 Belt 2 plies 2 plies 2 plies 2 plies 2 plies 2 plies Outer belt ply Monofilament cord Cord structure 1 × 1 1 × 1 1 × 1 1 × 1 1 × 1 × 0.55 1 × 1 × 0.42 Sectional shape oval oval oval oval circle circle Sectional area S (sq. mm) 0.139 0.139 0.139 0.139 0.238 0.139 Waving 2-D spiral 2-D spiral spiral Bend none none none none none none Pitch PA (mm) — 20 20 — 21 20 Amplitude HA (mm) — 0.18 0.18 — 0.19 0.18 HA/PA — 0.009 0.009 — 0.009 0.009 Pitch PB (mm) 20 — 20 20 — — Amplitude HB (mm) 0.5 — 0.18 0.18 — — HB/PB 0.025 — 0.009 0.009 — — Pitch PB2 (mm) — — — 20 — — Amplitude HB2 (mm) — — — 0.18 — — HB2/PB2 — — — 0.009 — — Bending rigidity M (g cm) 39 39 39 39 100 42 Cord count N (/5 cm) 40 40 40 40 28 40 S × N (cord weight index) 5.56 5.56 5.56 5.56 6.664 5.56 M × N (cord rigidity index) 1560 1560 1560 1560 2800 1680 Inner belt ply Multi-filament cord Cord structure 1 × 5 × 0.25 1 × 5 × 0.25 1 × 5 × 0.25 1 × 5 × 0.25 1 × 5 × 0.25 1 × 3 × 0.27 Sectional area S (sq. mm) 0.245 0.245 0.245 0.245 0.245 0.245 Bending rigidity M (g cm) 28 28 28 28 28 28 Cord count N (/cm) 35 35 35 35 35 35 S × N (cord weight index) 8.575 8.575 8.575 8.575 8.575 8.575 M × N (cord rigidity index) 980 980 980 980 980 980 Tire weight 97 96 96 96 96 96 Durability 240 89 89 89 79 88 Steering stability 7 10 10 10 9 9 Ride comfort 8 8 8 8 6 9

[0056] TABLE 3 Tire Prior 2 Ex. 11 Prior 3 Ex. 12 Tire size 205/85R16 205/85R16 11R22.5 11R22.5 Belt 3 plies 3 plies 4 plies 4 plies Outermost belt ply Multi-filament cord none none Cord structure 3 × 0.175 + 6 × 0.32 3 × 0.20 + 6 × 0.35 Monofilament cord none none Cord structure 1 × 1 × 0.60 1 × 1 × 0.60 Sectional shape — circle — circle Sectional area S (sq. mm) — 0.283 — 0.283 Waving spiral — spiral Bend — none — none Pitch PA (mm) — 28 — 28 Amplitude HA (mm) — 0.26 — 0.26 HA/PA — 0.009 — 0.009 Bending rigidity M (g cm) — 159 — 159 Cord count N (/5 cm) — 37 — 37 S × N (cord weight index) — 10.471 — 10.471 M × N (cord rigidity index) — 5883 — 5883 Inner belt plis Multi-filament cord Cord structure 3rd belt ply — — 3 × 0.20 + 6 × 0.35 3 × 0.20 + 6 × 0.35 2nd belt ply 3 × 0.175 + 6 × 0.32 3 × 0.175 + 6 × 0.32 3 × 0.20 + 6 × 0.35 3 × 0.20 + 6 × 0.35 Innermost belt ply 3 × 0.175 + 6 × 0.32 3 × 0.175 + 6 × 0.32 3 × 0.20 + 6 × 0.35 3 × 0.20 + 6 × 0.35 Section area S (sq. mm) 0.555 0.555 0.672 0.672 Bending rigidity M (g cm) 91 91 130 130 Cord count N (/5 cm) 26 26 26 26 S × N (cord weight index) 14.43 14.43 17.472 17.472 M × N (cord rigidity index) 2366 2366 3380 3380 Tire weight 100 96 100 98 Durability 100 64 100 73 Steering stability 7 9 7 8 Ride comfort 7 8 7 8

[0057]FIG. 1D shows a radial tire for passenger cars according to the present invention, wherein the carcass 6 comprises two plies 6 a and 6 b, and the belt 7 comprises at least two cross plies 7 a and 7 b all of which are a waved monofilament cord ply 20 as shown in FIGS. 14 and 17. That is, The belt 7 is composed of waved monofilament cords 10 (10A, 10B) only.

[0058] In this example, the belt 7 consists of two plies each made of the cords laid at an angle of from 15 to 30 degrees with respect to the circumferential direction of the tire.

[0059] In FIG. 14, each of the waved monofilament cord plies 20 is made of waved monofilament cords 10A having a circular cross sectional shape and waved spirally. The spiral pitches P are in a range of from 0.008 to 0.08 times, preferably 0.010 to 0.060 times the circumference W of the tire measured along the tire equator C under a standard inflated condition. And the height H of a wave which corresponds to the outer diameter of the spiral is preferably set in a range of from 0.002 to 0.025 times, preferably 0.010 to 0.020 times the pitch P of the wave. The ratio H/P of the wave height H to the pitch P of a wave is substantially constant in the tire circumferential direction. For example, in case of passenger tires, the spiral pitch P is 20 to 100 mm and the wave height H is 0.10 to 0.20 mm. In case of light-truck tires, the pitch P is 40 to 120 mm and the wave height H is 0.13 to 0.25 mm. In case of truck-bus tires, usually, the pitch P is 60 to 140 mm and the wave height H is 0.16 to 0.30 mm.

[0060] The above-mentioned standard inflated condition is that the tire is mounted on a standard wheel rim and inflated to a standard inner pressure but loaded with no tire load. The standard rim is the “standard rim” specified in JATMA, the “Measuring Rim” in ETRTO, the “Design Rim” in T&RA or the like. The standard pressure is the “maximum air pressure” in JATMA, the “Inflation Pressure” in ETRTO, the maximum pressure given in the “Tire Load Limits at Various Cold Inflation Pressures” table in T&RA or the like. In case of passenger car tires, however, 180 kPa is used as the standard pressure.

[0061] Further, the average thickness R of the waved monofilament cord 10 is set in a range of from 0.13×10⁻³ to 0.28×10⁻³ times, preferably 0.18×10⁻³ to 0.25×10⁻³ times the tire circumference W to provide a necessary rigidity for the belt. Usually, the average thickness R is 0.38 to 0.46 mm in case of passenger tires. In case of light-truck tires, the average thickness R is 0.46 to 0.54 mm. In case of truck-bus tires, the average thickness R is 0.54 to 0.70 mm. Here, the average thickness R means, 1) the diameter D in case the cord section is a circle, 2) the average (D1+D2)/2 of a thickness D1 along the major axis and a thickness D2 along the minor axis in case of a low aspect ratio cross section as shown in FIG. 16.

[0062] Besides the above-mentioned spiral waving, as shown in FIGS. 18 and 19, 2-D waving can be made for the waved monofilament cord 10. In this case, the cord 10 is waved in a plane parallel to the tread face as shown in FIG. 17. Curved waves not including a straight part such as sine curve as shown in FIG. 18 and zigzag waves including straight parts as shown in FIG. 19 can be used. The pitches P and wave heights H of the waves and the average cord thickness R are limited in the same manner as the spirally waved monofilament cord 10.

[0063] If the pitches P are less than 0.008 times the tire circumference W, steering stability deteriorates. If the wave height H is more than 0.025 times the pitch P, the strength and fatigue resistance of the cord decrease, and a ply separation failure becomes liable to occur. If the average thickness R is less than 0.13×10⁻³ times the tire circumference W, it becomes difficult to obtain a necessary cornering power. If the average thickness R is more than 0.28×10⁻³ times the tire circumference W, the cord durability decreases and ride comfort is deteriorated

[0064] Comparison test 2

[0065] Passenger car tires (size 175/70R13), light-truck tires (size 275/70R16) and truck-bus tires (size 11R22.5) having the structure shown in FIG. 1D and specifications shown in Tables 4 and 5 were made and tested in the same manner as above.

[0066] The results are shown in Tables 4 and 5. TABLE 4 Tire Prior 1 Ref. 1 Ref. 2 Ref. 3 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Tire size 175/70R13 175/70R13 175/70R13 175/70R13 175/70R13 175/70R13 175/70R13 175/70R13 Circumference W (mm) 1810 1810 1810 1810 1810 1810 1810 1810 Belt 2 plies 2 plies 2 plies 2 plies 2 plies 2 plies 2 plies 2 plies Cord structure 1 × 5 × 0.25 1 × 1 × 0.40 1 × 1 × 0.40 1 × 1 × 0.40 1 × 1 × 0.40 1 × 1 × 0.40 1 × 1 × 0.40 1 × 1 × 0.40 Waving none none Pitch P (mm) — 10 160 — 16 40 80 130 P/W — 0.006 0.088 — 0.009 0.022 0.044 0.072 Height H (mm) — 0.18 0.26 — 0.03 0.08 0.13 0.25 H/P — 0.018 0.0016 — 0.0019 0.002 0.0016 0.0019 Average thickness R (mm) 0.25 0.4 0.4 0.4 0.4 0.4 0.4 0.4 R/W — 0.00022 0.00022 — 0.00022 0.00022 0.00022 0.00022 Cord count/5 cm 30 40 40 40 40 40 40 40 Tire weight 100 92 92 92 92 92 92 92 Durability 100 85 83 80 100 101 100 100 Steering stability 7 5 8 7 7 8 9 9 Ride comfort 7 8 5 5 8 8 8 7

[0067] TABLE 5 Tire Ex. 5 Ex. 6 prior 2 Ref. 4 Ex. 7 prior 3 Ref. 5 Ex. 8 Tire size 175/70R13 175/70R13 275/70R16 275/70R16 275/70R16 11R22.5 11R22.5 11R22.5 Circumference W (mm) 1810 1810 2489 2489 2489 3306 3306 3306 Belt 2 plies 2 plies 2 plies 2 plies 2 plies 2 plies 2 plies 2 plies Cord structure 1 × 1 × 0.40 1 × 1 × 0.40 2 + 7 × 0.22 1 × 1 × 0.50 1 × 1 × 0.50 3 × 0.20 + 6 × 0.35 1 × 1 × 0.65 1 × 1 × 0.65 Waving none none Pitch P (mm) 80 80 — 15 100 — 20 130 P/W 0.044 0.044 — 0.006 0.04 — 0.006 0.039 Height H (mm) 0.13 0.13 — 0.18 0.18 — 0.24 0.24 H/P 0.0016 0.0016 — 0.012 0.0018 — 0.012 0.0018 Average thickness R (mm) 0.4 0.4 0.22 0.5 0.5 0.20 & 0.35 0.65 0.65 R/W 0.00022 0.00022 0.0002 0.0002 0.0002 0.0002 Cord count/5 cm 40 40 26 40 40 26 35 35 Tire weight 92 92 100 93 93 100 93 93 Durability 100 102 100 86 100 — — — Steering stability 9 9 7 5 9 7 6 9 Ride comfort 8 8 7 8 8 7 8 8 

1. A pneumatic tire comprising a carcass extending between bead portions through a tread portion and sidewall portions, and a belt disposed radially outside the carcass in the tread portion, said belt comprising a monofilament cord ply made of monofilament cords each made of a waved steel monofilament.
 2. The pneumatic tire according to claim 1 , wherein the belt further comprises at least one multi-filament cord ply made of multi-filament cords each made of a plurality of steel filaments twisted together.
 3. The pneumatic tire according to claim 2 , wherein the radially outermost ply of the belt is a monofilament cord ply.
 4. The pneumatic tire according to claim 1 , wherein the belt consists of said monofilament cord ply and at least one multi-filament cord ply made of multi-filament cords each made of a plurality of steel filaments twisted together.
 5. The pneumatic tire according to claim 4 , wherein the radially outermost ply of the belt is a monofilament cord ply.
 6. The pneumatic tire according to claim 1 , 2 , 3, 4 and 5, wherein each of the waved monofilament steel cords has a cross sectional area of from 0.09 to 0.30 sq.mm.
 7. The pneumatic tire according to claim 1 , 2 , 3, 4 and 5, wherein each of the waved monofilament steel cord is spirally waved so that the wave pitches PA are in a range of from 14 to 50 mm, and the amplitude HA of each wave is in a range of from 0.002 to 0.02 times the pitch PA of the wave.
 8. The pneumatic tire according to claim 1 , 2 , 3, 4 and 5, wherein each of the waved monofilament steel cord is two-dimensionally waved so that the wave pitches PB are in a range of from 5 to 50 mm, and the amplitude HB of each wave is in a range of from 0.002 to 0.05 times the pitch PB of the wave.
 9. The pneumatic tire according to claim 1 , wherein the belt further comprises at least one monofilament cord ply made of monofilament cords each made of a waved steel monofilament.
 10. The pneumatic tire according to claim 1 , wherein the belt consists of said monofilament cord ply and further at least one monofilament cord ply made of monofilament cords each made of a waved steel monofilament.
 11. The pneumatic tire according to claim 1 , 9 or 10, wherein each of the waved monofilament steel cords is waved so that the wave pitches P are in a range of from 0.008 to 0.08 times the circumference W of the tire, and the wave height H of each wave is in a range of from 0.002 to 0.025 times the pitch P of the wave.
 12. The pneumatic tire according to claim 1 , 9 , or 10, wherein each of the waved monofilament steel cords has an average thickness of from 0.13×10^(−3 to) 0.28×10⁻³ times the circumference W of the tire.
 13. The pneumatic tire according to claim 1 , 9 or 10, wherein each of the waved monofilament steel cords is waved so that the wave pitches P are in a range of from 0.008 to 0.08 times the circumference W of the tire, and the wave height H of each wave is in a range of from 0.002 to 0.025 times the pitch P of the wave, and each of the waved monofilament steel cords has an average thickness of from 0.13×10⁻³ to 0.28×10⁻³ times the circumference W of the tire. 